Ocean sunfish

The ocean sunfish, Mola mola, or common mola, is the heaviest known bony fish in the world. It has an average adult weight of 1,000 kg (2,200 lb). The species is native to tropical and temperate waters around the globe. It resembles a fish head with a tail, and its main body is flattened laterally. Sunfish can be as tall as they are long when their dorsal and ventral fins are extended.

Sunfish live on a diet that consists mainly of jellyfish, but because this diet is nutritionally poor, they consume large amounts in order to develop and maintain their great bulk. Females of the species can produce more eggs than any other known vertebrate.[1] Sunfish fry resemble miniature pufferfish, with large pectoral fins, a tail fin and body spines uncharacteristic of adult sunfish.

Adult sunfish are vulnerable to few natural predators, but sea lions, orcas and sharks will consume them. Among humans, sunfish are considered a delicacy in some parts of the world, including Japan, the Korean peninsula and Taiwan. In the EU, regulations ban the sale of fish[2] and fishery products[3] derived of the Molidae family. Sunfish are frequently, though accidentally, caught in gillnets.

Many of the sunfish's various names allude to its flattened shape. Its specific name, mola, is Latin for "millstone", which the fish resembles because of its grey colour, rough texture, and rounded body. Its common English name, sunfish, refers to the animal's habit of sunbathing at the surface of the water. The Dutch-, Portuguese-, French-, Catalan-, Spanish-, Italian-, Russian- and German-language names, respectively maanvis, peixe lua, poisson lune, peix lluna, pez luna, pesce luna, рыба-луна and Mondfisch, mean "moon fish", in reference to its rounded shape. In German, the fish is also known as Schwimmender Kopf, or "swimming head". In Polish it is named samogłów, meaning "head alone", because it has no true tail. The Chinese translation of its academic name is fan-che yu 翻車魚, meaning "toppled car fish". The ocean sunfish has various superseded binomial synonyms, and was originally classified in the pufferfish genus, as Tetraodon mola.[4][5] It is now placed in its own genus, Mola, with two species: Mola mola and Mola ramsayi. The ocean sunfish, Mola mola, is the type species of the genus.[6]

The Molidae family belongs to the orderTetraodontiformes, which includes pufferfish, porcupinefish and filefish. It shares many traits common to members of this order, including the four fused teeth that form the characteristic beak and give the order its name (tetra=four, odous=tooth, and forma=shape). Indeed, sunfish fry resemble spiky pufferfish more than they resemble adult molas.[7]

A taxidermied ocean sunfish, caught in 1890 off the former Austrian Adriatic coast.

The caudal fin of the ocean sunfish is replaced by a rounded clavus, creating the body's distinct truncated shape. The body is flattened laterally, giving it a long oval shape when seen head-on. The pectoral fins are small and fan-shaped, while the dorsal fin and the anal fin are lengthened, often making the fish as tall as it is long. Specimens up to 3.2 m (10.5 ft) in height have been recorded.[8]

A drawing of an ocean sunfish from 1887.

The mature ocean sunfish has an average length of 1.8 m (5.9 ft), a fin-to-fin length of 2.5 m (8.2 ft) and an average weight of 1,000 kg (2,200 lb),[1] although individuals up to 3.3 m (10.8 ft) in length[8] 4.2 m (14 ft) across the fins[9] and weighing up to 2,300 kg (5,100 lb)[10] have been observed.

The spinal column of M. mola contains fewer vertebrae and is shorter in relation to the body than that of any other fish.[11] Although the sunfish descended from bony ancestors, its skeleton contains largely cartilaginous tissues, which are lighter than bone, allowing it to grow to sizes impractical for other bony fishes.[11][12] Its teeth are fused into a beak-like structure,[10] and pharyngeal teeth located in the throat.[13]

In the course of its evolution, the caudal fin (tail) of the sunfish disappeared, to be replaced by a lumpy pseudo-tail, the clavus. This structure is formed by the convergence of the dorsal and anal fins.[15][16] The smooth-denticled clavus retains twelve fin rays, and terminates in a number of rounded ossicles.[17] Without a true tail to provide thrust for forward motion and equipped with only small pectoral fins, Mola mola relies on its long, thin dorsal and anal fins for propulsion, driving itself forward by moving these fins from side to side.[18][citation needed]

Ocean sunfish often swim near the surface, and their protruding dorsal fins are sometimes mistaken for those of sharks.[19] However, the two can be distinguished by the motion of the fin. Sharks, like most fish, swim by moving the tail sideways while keeping the dorsal fin stationary. The sunfish, on the other hand, swings its dorsal fin and anal fin in a characteristic sculling motion which can be used to identify it.[20]

Adult sunfish range from brown to silvery-gray or white, with a variety of mottled skin patterns; some of these patterns may be region-specific.[1] Colouration is often darker on the dorsal surface, fading to a lighter shade ventrally as a form of counter-shading camouflage. Mola mola also exhibits the ability to vary skin colouration from light to dark, especially when under attack.[1] The skin, which contains large amounts of reticulated collagen, can be up to 3 in (7.6 cm) thick on the ventral surface, and is covered by denticles and a layer of mucus instead of scales. The skin on the clavus is smoother than that on the body, where it can be as rough as sandpaper.[11]

More than 40 species of parasites may reside on the skin and internally, motivating the fish to seek relief in a number of ways.[1][17] In temperate regions, drifting kelp fields harbour cleaner wrasses and other fish which remove parasites from the skin of visiting sunfish. In the tropics, the mola solicits cleaning help from reef fishes. By basking on its side at the surface, the sunfish also allows seabirds to feed on parasites from its skin. Sunfish have been reported to breach, clearing the surface by more than three body lengths, possibly as another effort to dislodge parasites.[19][21]

Although early research suggested that sunfish moved around mainly by drifting with ocean currents, individuals have been recorded swimming 26 km in a day, at a top speed of 3.2 km/h.[23] Sunfish are pelagic and swim at depths of up to 600 m (2,000 ft). Contrary to the general perception that sunfish spend much of their time basking at the surface, research suggests that adult M. mola actually spend a large portion of their lives submerged at depths greater than 200 m (660 ft), occupying both the epipelagic and mesopelagic zones.[24]

Sunfish are most often found in water warmer than 10 °C(50 °F);[24] prolonged periods spent in water at temperatures of 12 °C(54 °F) or lower can lead to disorientation and eventual death.[20] Researchers theorize that surface basking behaviour, in which a sunfish swims on its side, presenting its largest profile to the sun, may be a method of "thermally recharging" following dives into deeper, colder water.[22][25] Others point to sightings of the fish in colder waters outside of its usual habitat, such as those southwest of England, as evidence of increasing marine temperatures.[26][27]

Sunfish are usually found alone, but occasionally in pairs or in large groups while being cleaned.[11] They swim primarily in open waters, but are sometimes seen near kelp beds taking advantage of resident populations of smaller fish which remove ectoparasites from their skin. Because sunfish must consume a large volume of prey, their presence in a given area may be used as an indicator of nutrient-rich waters where endangered species may be found.[11]

The diet of the ocean sunfish consists primarily of various jellyfish. It also consumes salps, squid, crustaceans, small fish, fish larvae, and eel grass.[1] This range of food items indicates that the sunfish feeds at many levels, from the surface to deep water, and occasionally down to the seafloor in some areas.[1] The diet is nutritionally poor, forcing the sunfish to consume a large amount of food to maintain its size.[20]

Ocean sunfish may live up to ten years in captivity, but their lifespan in a natural habitat has not yet been determined.[19] Their growth rate is also undetermined. However, it is known that a young specimen at the Monterey Bay Aquarium increased in weight from 26 to 399 kg (57 to 880 lb) and reached a height of nearly 1.8 m (5.9 ft) in fifteen months.[20]

The sheer size and thick skin of an adult of the species deters many smaller predators, but younger individuals are vulnerable to predation by bluefin tuna and mahi mahi. Adults are consumed by sea lions, orcas and sharks.[11] Sea lions appear to hunt sunfish for sport, tearing the fins off, tossing the body around, and then simply abandoning the still-living but helpless fish to die on the seafloor.[1][20]

The mating practices of the ocean sunfish are poorly understood, but spawning areas have been suggested in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Oceans.[11] Females can produce as many as 300 million eggs at a time, more than any other known vertebrate.[1] Sunfish eggs are released into the water and externally fertilized by sperm.[17]

Newly hatched sunfish larvae are only 2.5 mm (0.098 in) long. They grow to become fry, and those which survive grow many millions of times their original size before reaching adult proportions.[18] Sunfish fry, with large pectoral fins, a tail fin and body spines uncharacteristic of adult sunfish, resemble miniature pufferfish, their close relatives.[17][28] Young sunfish school for protection, but this behaviour is abandoned as they grow.[29]

Despite their size, ocean sunfish are docile, and pose no threat to human divers.[17] Injuries from sunfish are rare, although there is a slight danger from large sunfish leaping out of the water onto boats; in one instance a boy was knocked off his boat when a sunfish leaped onto it.[30] Areas where they are commonly found are popular destinations for sport dives, and sunfish at some locations have reportedly become familiar with divers.[10] The fish is more of a problem to boaters than to swimmers, as its immense size and weight can cause significant damage to a boat that strikes one of these fish. Collisions with sunfish are very common in some parts of the world and have caused damage to the hull of a boat,[31] and their bodies can become lodged in the propellers of larger ships.[17]

A dish made with the meat of the ocean sunfish

The meat of the ocean sunfish is considered a delicacy in some regions, the largest markets being Taiwan and Japan. All parts of the sunfish are used in cuisine, from the fins to the internal organs.[14] Some parts of the fish are used in some areas of traditional medicine.[10] If the body does contain toxins, then the marketing and sale of ocean sunfish meat is forbidden in the European Union according to Regulation (EC) No 853/2004 of the European Parliament and of the Council.[3]

Sunfish are accidentally but frequently caught in drift gillnet fisheries, making up nearly 30% of the total catch of the swordfish fishery employing drift gillnet in California.[18] The by-catch rate is even higher for the Mediterranean swordfish industry, with 71% to 90% of the total catch being sunfish.[14][29]

The fishery by-catch and destruction of ocean sunfish are unregulated worldwide. In some areas, the fish are "finned" by fishermen who regard them as worthless bait thieves; this process, in which the fins are cut off, results in the eventual death of the fish, because it can no longer propel itself without its dorsal and anal fins.[32] The species is also threatened by floating litter such as plastic bags which resemble jellyfish, its main food. Bags can choke and suffocate an individual or fill its stomach to the extent that it starves.[19]

Many areas of sunfish biology remain poorly understood, and various research efforts are underway, including aerial surveys of mola populations,[33] satellite surveillance using pop-off satellite tags,[14][33] genetic analysis of tissue samples,[14] and collection of amateur sighting data.[34] Recent studies indicate a decrease in sunfish populations that may be caused by more frequent bycatch and the increasing popularity of sunfish in human diet.[11]

While it is claimed that the first ocean sunfish to be held in an aquarium in the United States arrived at the Monterey Bay Aquarium in August 1986,[39] other specimens have previously been held at other locations. Marineland of the Pacific, located on the Palos Verdes Peninsula in Los Angeles County, California, held an ocean sunfish in its warm water tank as early as 1957,[40] and in 1964 held a 650-pound specimen, claimed as the largest ever captured at that point in time.[41] However, another 1000-pound specimen was brought alive to Marineland Studios Aquarium, near St. Augustine, Florida, in 1941.[42] As of 2012[update], Monterey Bay Aquarium is the only location in the United States where the sunfish is displayed.[43] Because sunfish had not been kept in captivity on a large scale before, the staff at Monterey Bay were forced to innovate and create their own methods for capture, feeding, and parasite control. By 1998, these issues were overcome, and the aquarium was able to hold a specimen for more than a year, later releasing it after its weight increased by more than 14 times.[20]Mola mola have since become a permanent feature of the Open Sea exhibit.[18] Monterey Bay Aquarium's largest sunfish specimen was euthanized on February 14, 2008 after an extended period of poor health.[44]

A major concern to curators is preventative measures taken to keep specimens in captivity from injuring themselves by rubbing against the walls of a tank, since ocean sunfish cannot easily maneuver their bodies.[35] In a smaller tank, hanging a vinyl curtain has been used as a stopgap measure to convert a cuboid tank to a rounded shape and prevent the fish from scraping against the sides. A more effective solution is simply to provide enough room for the sunfish to swim in wide circles.[20] The tank must also be sufficiently deep to accommodate the vertical height of the sunfish, which can be nearly as tall as it is long, and may reach a height of 3.2 m (10 ft).[8]

Feeding captive sunfish in a tank with other faster-moving, more aggressive fish can also present a challenge. Eventually, the fish can be taught to respond to a floating target to be fed,[43] and to take food from the end of a pole or from human hands.[20]